Abundance of New Unfamiliar React APIs (Profiler, ViewTransition, cache, etc.)
4React 19 introduced numerous new APIs with low familiarity among developers: Profiler (57% unfamiliar), ViewTransition (41%), Activity (41%), cache (41%), useEffectEvent (40%), useDeferredValue (39%). This creates knowledge gaps and increases the learning curve for developers trying to stay current.
docsReactProfilerViewTransition+3